In an urban environment, vehicular emissions usually constitute the largest source of ultrafine particles (UFPs, diameter <0.1 m) and strong evidence from toxicological studies has demonstrated that UFPs are more toxic per unit mass than regulated larger particles. Taxi drivers are expected to be exposed to high levels of UFPs because of their close proximity to the source and prolonged exposure time. However, there is a critical data and knowledge gap on taxi drivers' exposure to UFPs at work and even less knowledge about the influencing factors leading to high exposures. Lack of such knowledge is an important problem because until reliable data and knowledge regarding exposure causes and correlates are available, quantitative UFP exposure/risk assessments cannot be completed and appropriate UFP mitigation policies cannot be set. Our long-term goal is to reduce occupational diseases attributable to exposure to UFPs at workplace. The objective of this proposal is to develop UFP exposure assessment instrument and explore novel low-cost UFP exposure mitigation strategies for taxi drivers. The central hypothesis is that 1) vehicle characteristics and driving conditions will explain a significant portion of the total variability f taxi drivers' exposure to UFPs; and 2) replacing commercially installed vehicle cabin filters with high efficiency particulate air (HEPA) filters will significantly re- duce taxi drivers' exposure t UFPs. Guided by preliminary data, this hypothesis will be tested by pursuing two specific aims: 1) develop exposure assessment instrument by identifying major factors that influence UFP concentrations inside taxi cabins; and 2) develop novel low-cost exposure mitigation strategies to reduce taxi drivers' exposure to UFPs. This will be the first systematic study to assess UFP exposures among taxi drivers in the US and the first study to explore cost-effective techniques to reduce UFP levels inside taxicabs. This study will benefit both taxi drivers, who normally spend 72 hours per week at work, and susceptible passengers, such as elderly and people with disabilities who require taxi service more frequently. We will monitor UFP and other vehicular emitted pollutant levels simultaneously inside and outside 40 taxis, build statistical models to predict in-cabin UFP levels, and test to what extent retrofitting commercially installed vehicle cabin air filters with HE- PA filters will reduce in-cabin UFP levels. The rationale for the proposed research is that the developed expo- sure model will facilitate subsequent health effect studies and the identified mitigation strategies will result in new and innovative approaches to prevent taxi drivers from high UFP exposures at work. The proposed re- search is innovative because it represents a new and substantive departure from the status quo by focusing on UFP exposure among taxi drivers in the occupational setting and developing novel low-cost technologies to re- duce their exposure. The proposed research is significant because it is the first step in a continuum of research that identifies workers at risk, protects workers fom high exposures, reveals the health effects of UFPs in the occupational setting, and ultimately reduces the workplace burden of diseases attributable to UFPs.